The invention relates to a data receiver circuit for infrared signals comprising a plurality of infrared detectors the signals of which are supplied to a summing circuit to increase the useful power while suppressing interfering signals.
Data transmitter and data receiver circuits for infrared signals are known e.g. with TV sets for program selection, volume adjustment etc. Also for toll systems on roads, transmitters in the infrared range comprising semiconductor light transmitting elements, such as luminescence diodes or laser diodes, have been suggested instead of radiofrequency transmitters in the decimeter or centimeter wave length range. Such infrared data transmission systems can be operated with the following types of modulation:
In the data receiver circuit, germanium or silicon photodiodes are used as infrared detectors, the resulting xe2x80x9cphoto(electric) currentxe2x80x9d being amplified and evaluated. If such data receiver circuits are used in surroundings having little background light, e.g. in the interior of a building, in the shade or only during the night, simple amplifiers and compensation circuits will suffice to suppress the background photo current. With increasing background photo current, in particular during operation of the receivers in direct sunlight, these earlier circuit techniques do, however, give rise to problems and finally become useless, since the high background photo current i.a. causes a high current noise in which the useful signal currents, commonly in the nA range, get lost.
From AT 376 083 B, a circuit for suppressing electromagnetic interferences in infrared receiver devices is known. There, two groups of photodiodes connected in parallel are provided, the one diodes being connected to a positive supply voltage via a series resistor R1, and the other diodes being connected to ground via a series resistor. From the one group of photodiodes, useful signals of negative polarity are supplied via a capacitor to one input of a difference amplifier, whereas the useful signals of positive polarity are supplied via a capacitor to the other input of the difference amplifier. In this manner, the useful signals should occur summed at the output of the difference amplifier, whereas an interfering radiation, coming, e.g., from radio stations, is suppressed. In a modified embodiment, additional operational amplifiers are provided between the capacitors and the inputs of the difference amplifier DV, wherein these operation amplifiers, as so-called transimpedance amplifiers, are to enable more rapid signal processing.
Also in the circuit shown in U.S. Pat. No. 5,355,242 A for receiving infrared signals, two groups of photodiodes connected in parallel, cf. FIG. 1, are provided, the useful signals of which are supplied via transformers and capacitors to transimpedance amplifiers; the secondary windings of the transformers are reversed in phase so as to obtain useful signals of positive and negative polarities whichxe2x80x94similar to AT 376 083 Bxe2x80x94are supplied via transimpedance amplifiers after having passed a band pass filter. The outputs of the difference amplifier are connected to the inputs of a further difference amplifier via low-pass filters, a gate circuit acting as a noise barrier being connected to the outputs of the further difference amplifier. Thus, summing of the useful signals in this circuit occurs at the connection point of the parallel-connected photodiodes with the primary winding of the respective transformer.
In the known circuits it is disadvantageous that if several infrared detectors (photodiodes) are connected in parallel to each other to increase the useful power, also the parallel capacity of the diodes is increased, whereby in turn the switching rise times and switching decay times are increased.
It is an object of the invention to provide a data receiver circuit of the initially mentioned type in which short switching rise times and short switching decay times are achieved, on the one hand, and in which the useful data signal/noise ratio is high despite a high current noise caused by an intensive background light.
In a data receiver circuit of the above-defined kind, this object is achieved according to the invention in that several input circuits, each having an infrared detector and an associated separate amplifier, are provided, and these input circuits are connected in parallel to the summing circuit. In this manner, the amplifiers act signal-synchronously, yet they have spontaneous noise phases which partially add up and partially cancel themselves out in the summing circuit, according to the law of probability. In contrast, the useful signal is added up linearly with equal phase in the summing circuit. On the whole, thus, the signal/noise ratio increases constantly with the number of amplifiers used.
According to an advantageous embodiment of the invention it is provided that the output of the summing circuit is connected to the input of a comparator the other input of which is connected to the output of a threshold value generator, the input of which is connected to the output of the summing circuit. In this manner, the function of an AGC (automatic gain control) amplifier circuit is simulated in that useful signals of weak power are suppressed, or, if several transmitters are present, such as, e.g., at the toll site in a toll system on a road, only the transmitter transmitting with the highest power is received; in toll systems comprising several lanes and corresponding transmitters, this will be of particular importance with a view to a xe2x80x9clane selectivityxe2x80x9d.
In this connection it is also suitable if a main amplifier is provided between the output of the summing circuit and the one input of the comparator as well as the input of the threshold value generator. By this measure, the dynamic range for further processing of the useful signal can be increased.
For the automatic threshold value adjustment, it is furthermore particularly advantageous that, by rectifying and smoothing the output signal of the summing circuit and, optionally, of the main amplifier, the threshold value generator forms a sum of the peak value and the mean value of this signal in a pre-determined ratio, e.g. 1:2, which sum is delivered as a threshold value signal at the output of the threshold value generator.
With a view to a particularly suitable, simple and rapidly reacting embodiment of the amplifier it is suitable if the amplifiers of the input circuits are transistor amplifiers. The transistors in that instance may be simple bipolar transistors.
To keep the summing circuit as simple and cheap as possible, it has furthermore proven advantageous if the summing circuit is formed with capacitors which are connected to the respective amplifier output, on the one hand, and to a common connection point, on the other hand.
An embodiment which is particularly preferred due to the simple configuration of the input circuits is obtained if the infrared detectors are photodiodes which are connected in parallel to the emitter resistors of transistors; advantageously, the capacitors of the summing circuit are connected to the collector resistors of the transistors.
Finally, to form the threshold value, it has proven suitable with a view to short reaction times and a simple realization thereof if the input of the threshold value generator is connected to ground via a diode and a resistor arranged in series, that the connection point between the diode and the resistor is connected to the base of a transistor to which a capacitor storing the peak value of the supplied output signal of the summing circuit and, optionally, of the main amplifier is connected, and that a further capacitor is connected to the diode, or to the capacitor, respectively, to obtain the respective threshold value via a voltage divider circuit.